The present invention relates to a high-speed, large-capacity AC variable speed motor drive apparatus.
Motors can be primarily classified into DC motors and AC motors.
DC motors have small torque ripples and good controllability, and can be easily handled. DC motors have been used in a wide range of applications, in combination with DC chopper (thyristor chopper) devices. However, DC motors require extensive, and therefore time-consuming maintenance work on their brushes and commutators, and there are limitations on their maximum operation speed and/or their maximum capacity. For this reason, in recent years, DC motors have tended to be increasingly replaced by AC variable speed motors.
DC train systems also have the same limitations as outlined above, and drive systems using AC variable speed motors have been used gradually with the aim of high-speed operation and a large capacity.
Typical AC motors include induction motors and synchronous motors. Although AC motors also include reluctance motors and hysteresis motors, these have a considerably narrower range of applications.
A commutatorless motor is known, in which a counter electromotive force of a synchronous motor is used to naturally commutate a thyristor inverter. Since the commutatorless motor utilizes natural commutation, it can easily have a large capacity, has similar controllability to that of DC motors, and can be used in various fields. However, since the commutatorless motor requires a field pole, the overall motor device becomes bulky, and has a small overload strength due to the limitations thus imposed on natural commutation.
An inductance motor, in particular, a squirrelcage induction motor, has a simple and rigid structure, and can be easily handled. However, this motor requires a self-excited inverter, and has certain limitations due to the characteristics of the inverter.
Nowadays, self-extinction elements such as transistors, GTOs, and the like tend to have a large capacity, and are used in the self-excited inverter. In particular, a pulse-width modulation (PWM) controlled inverter can supply a sine wave current to a motor. Therefore, an AC variable speed motor having low noise and a small torque ripple can be realized. Meanwhile, various control techniques have been established for induction motors, such as a V/f=constant control, slip frequency control, vector control, and the like, such control techniques enabling characteristics equivalent to those of DC motors to be obtained.
A cycloconverter is known as a typical example, which utilizes a voltage from an AC power source to effect natural commutation. The cycloconverter can supply a sine wave current to a motor, and its capacity can be easily increased, due to its natural commutation. In particular, as is described in U.S. Pat. No. 4,418,380 (Nov. 29, 1983), U.S. Pat. No. 4,570,214 (Feb. 11 1983), or Japanese Patent Publication No. 59-14988, a reactive-power compensation type cycloconverter, in which an input power factor at a receiving end is controlled to be always 1, has received a great deal of attention.
The above types of AC motor drive techniques have been used in various fields of application, while taking their advantages. However, an apparatus for driving a high-speed, large-capacity motor, applied to DC train systems, cannot easily be realized by use of conventional techniques. More specifically, although the cycloconverter utilizes natural commutation so that its capacity can be easily increased, the output frequency of a cycloconverter is low and not suitable for high-speed operation.
An apparatus for driving an induction motor by means of a self-excited inverter requires large-capacity, self-extinction elements such as transistors, GTOs, and the like. Consequently, the resulting apparatus is expensive, and it is difficult to increase its capacity. An available switching frequency of the large-capacity, self-extinction elements (particularly the GTOs) is at most 1 kHz. If PWM control is applied to such elements, the output frequency of the self-excited inverter is at most 100 Hz.
Since the commutatorless motor utilizes natural commutation, its capacity can be easily increased, and high-speed operation is easy to achieve. However, the motor itself is complicated and bulky. Further, since a rectangular current is supplied to an armature winding, torque ripples produced by the motor are increased. In addition, problems associated with the way of commutation at the beginning of energization and with an insufficient overload strength, still remain.
On the other hand, along with an increase in the capacity of the motor, the influence of reactive power generated from the power source and that of harmonic components of the reactive power cannot be ignored. Variations in reactive power cause variations in the power source system voltage, and adversely influence other electrical equipment connected to the same power source system. A harmonic current induces induction problems in television systems, radio receivers, or communication lines, and harmonic components of the 3rd, 5th, and 7th orders are hard to remove, and therefore require appropriate countermeasures.
In contrast to this, the reactive-power compensation type cycloconverter is an effective means for solving the reactive power problem, and serves as a power converter for maintaining the input power factor of the receiving end always at 1. However, depending on the output frequency, a harmonic current appears at the input side, and countermeasures must be taken thereagainst. However, according to the current technique, the countermeasure is difficult to attain.
Recently, a power converter has been proposed, which has functions of both an AC power converter and an active filter, as disclosed in Japanese Patent Disclosure (Kokai) No. 59-61475. An AC motor drive system constituted by a combination of this power converter and a self-excited inverter has received a great deal of attention.
In the system utilizing the active filter, since an input current is controlled to be a sine wave in the same phase as that of the power source voltage, a harmonic component is small, and the input power factor can always be maintained to be 1. However, the converter must be constituted by self-extinction elements such as transistors and GTOs, and thus, a large-capacity system is difficult to realize and has an economical problem.